1. Field of the Invention
The present invention relates to a rotary joint device in which a fluid channel of an outer cylinder and a fluid channel of a rotating shaft communicate with each other, and in particular, to a rotary joint device including an outer cylinder that is formed by combining a plurality of cylindrical blocks in an axial direction.
2. Description of the Related Art
Such rotary joint devices are widely used to connect pipes and apparatuses that rotate relative to each other. For example, such rotary joint devices are built in a main shaft driving apparatus that is mounted in a machine tool and that includes a main shaft that supports a device to be rotated and that is rotated by a driving device. The rotary joint devices are used to supply and drain a coolant, a hydraulic fluid, or the like between the machine tool and the device to be rotated.
Japanese Examined Utility Model Registration Application Publication No. 7-54712 (Patent Document 1) discloses an example of such rotary joint devices. The rotary joint device disclosed in Patent Document 1 is an outer-cylinder-connected-type rotary joint device in which an outer cylinder is fitted onto a rotating shaft. The outer cylinder includes a plurality of fixed cylindrical bodies (which correspond to “cylindrical blocks” in the present invention) that are divided in accordance with the types of supplied fluids and that are connected to each other. Seal rings are interposed between the fixed cylindrical bodies and the rotating shaft (strictly speaking, an inner sleeve that is fixed to the rotating shaft and that rotates together with the rotating shaft) to prevent leakage of fluids between the fixed cylindrical bodies.
As described above, a rotary joint device is used in a machine tool by connecting a rotating shaft of the rotary joint device to a supply-side main shaft that is rotated by a driving device. In this case, when the outer cylinder and the rotating shaft rotate relative to each other, friction between seal members and the rotating shaft generates rotational resistance, and thereby a load is applied to the driving device that rotates the main shaft connected to the rotating shaft. In particular, in the case of an outer-cylinder-connected-type rotary joint apparatus as disclosed in Patent Document 1, a high load is applied to the driving device because a seal ring is provided for each port and a mechanical seal is provided for each fixed cylindrical body so that there are a large number of seal members.
Therefore, the driving device needs to be designed in consideration of such a load and a motor having a high output power needs to be used as a drive source. Accordingly, the driving device is increased in size. As the driving device and the motor are increased in size, the manufacturing cost increases.
Some driving devices include, for example, a direct drive motor (so-called DD motor) as the drive source. The DD motor has a motor rotor that is disposed coaxially with the main shaft and directly connected to the main shaft, and the DD motor rotates the main shaft without using a speed- reducing device such as gears or the like. Therefore, a load applied to the driving apparatus due to the rotational resistance of the rotary joint device acts on the DD motor without being reduced by the speed-reducing device such as gears or the like. Thus, it is necessary to use a DD motor having a high output power in accordance with the load.
The higher the output power of a DD motor, the larger the outside diameter and the length of the DD motor. However, there are limitations on the outside diameter and the length of the DD motor because the driving apparatus is mounted in a machine tool having a limited installation space. Therefore, the output power of a usable DD motor is limited, so that it is necessary to reduce a load applied to the driving apparatus by reducing the rotational resistance of the rotary joint device.
In the case of the outer-cylinder-connected-type rotary joint apparatus disclosed in Patent Document 1, a high load is applied to the driving apparatus because there are a large number of seal members, which are provided for every port. Therefore, in order to reduce a load applied to the driving apparatus, it is preferable to reduce the number of the seal members provided for every port of the fixed cylindrical body or to eliminate the seal members. However, if simply the number of the seal members is reduced or the seal members are eliminated, a large amount of fluid leaks through the clearance between an inner peripheral surface of the outer cylinder and the rotating shaft to the outside of a fluid channel of the port through which the fluid is supplied. Therefore, the pressure of the supplied fluid cannot be maintained at the pressure needed by a device to be rotated.
In order to further reduce a load applied to the driving apparatus, an oil seal or the like that causes a lower friction may be used instead of a mechanical seal provided for each fixed cylindrical body. However, the oil seal or the like cannot prevent leakage of a fluid between the fixed cylindrical bodies, because the oil seal or the like cannot resist the pressure of the fluid that leaks through the clearance. As a result, different types of fluids in adjacent fixed cylindrical bodies become mixed with each other and form an emulsion, so that the fluid cannot be reused after being recovered.
The clearance described above (so-called diametrical clearance) may be reduced in order to maintain the pressure of a supplied fluid at the pressure needed by the device to be rotated and to suppress leakage of the fluid between the fixed cylindrical bodies. The pressure needed by the device to be rotated is, for example, a hydraulic pressure or a pneumatic pressure in the range of about 3.5 to 6.9 MPa. Needless to say, it is necessary to supply the fluid to the rotary joint device with a pressure higher than this. For example, by making the clearance be equal to or smaller than 0.005 mm, various fluids supplied to the ports can be separated from one another in a nearly liquid-tight or airtight manner.
However, if the diametrical clearance of the outer-cylinder-connected-type rotary joint apparatus is reduced, it becomes very difficult to assemble the fixed cylindrical bodies of the outer cylinder. This is particularly the case when reassembling the rotary joint apparatus such that the clearance between the fixed cylindrical bodies and the rotating shaft becomes the same as that before being disassembled for maintenance or the like. If the clearance is not made the same as that before disassembling, the amount of fluid that leaks through the clearance to the outside of the fluid channel increases, and the pressure of the fluid cannot be maintained at the pressure needed by the device to be rotated. Moreover, if the fixed cylindrical body and the rotating shaft interfere with each other, rotational resistance due to the interference causes a high load for the driving apparatus and prevents the driving apparatus from smoothly rotating the device to be rotated. Furthermore, due to the interference, abrasion or scoring occurs between the outer cylinder and the rotating shaft of the rotary joint device, which causes the rotary joint device to malfunction.